Researches on an optical imaging apparatus that irradiates an object, such as a biological body, with light from a light source, such as laser, and images information within the object obtained based on incident light have been active in the medical field. Photoacoustic imaging (PAI) is one of such optical imaging techniques. In photoacoustic imaging, an object is irradiated with pulsed light generated from a light source, acoustic waves (typically ultrasound waves) generated from an object tissue that has absorbed energy of the pulsed light propagated/diffused within the object is received, and object information is imaged based on the received signal.
When the object is irradiated with light, an object segment that has absorbed light energy momentarily expands and generates acoustic waves (called photoacoustic waves), due to the difference in absorption rate of light energy between a target segment, such as tumor, and other tissues. In photoacoustic imaging, photoacoustic waves generated by this photoacoustic effect are received utilizing a probe (receiving element).
By performing analytical processing of the received signal mathematically, information within the object, particularly initial sound pressure distribution, light energy absorption density distribution, absorption coefficient distribution, or the like, can be acquired. Such information can also be utilized in quantitative measurement of a particular substance within the object such as, for example, blood oxygen saturation. In recent years, preclinical researches on imaging of a blood vessel image of a small animal using photoacoustic imaging or clinical researches on applying this principle to diagnosis of breast cancer or the like have been active (NPL 1).
NPL 2 describes a testing apparatus that acquires information of an object, using a sensor in which receiving surfaces of a plurality of receiving elements are arranged on the inner surface of a hemispherical supporting unit. Since photoacoustic waves generated in a particular region can be received at a high sensitivity with the sensor, the resolution of object information in the particular region is high.
In the testing apparatus in NPL 2, the inside of the hemispherical supporting unit is filled with an acoustic matching member formed of liquid or gel for propagating photoacoustic waves to an ultrasound probe. It is indicated that information of an object with high resolution is acquired in a wide range by moving the position of the sensor in the horizontal direction with respect to the object immersed in the acoustic matching member.
A container-shaped sensor inside which a liquid acoustic matching member is held can be utilized not only for photoacoustic imaging but also for ultrasound echo diagnosis. In the case of using such a sensor, it is preferable that an acoustic matching member fill the gap between an object and the sensor and the sensor and the object be coupled acoustically.